1. Field of the Invention
This invention relates to an electrical double layer capacitor device and, more particularly, to a device formed by a plurality of electrical double layer capacitors connected in series.
2. Description of the Prior Art
An electrical double layer capacitor device of the type described has a low resistance and a large capacity. Because the electrical double layer capacitor device does not cause any chemical reaction to occur unlike a battery, its deterioration is very small due to repetition of charging and discharging. Therefore, the device can be often used as an accumulator free of maintenance. Further, because the electrical double layer capacitor device contains no harmful substance such as heavy metal as its component material, it is no harmful to the environment. Using this advantage, the electrical double layer capacitor device has been widely used as a memory backup device. Generally, the electrical double layer capacitor is connected to a battery or a power source obtained by converting commercial AC power to DC power, in parallel. Electric charge accumulated in the electrical double layer capacitor backs up various parts when the power source happens to be interrupted. Further, the electrical double layer capacitor device has been developed recently which has a tremendously large capacity, so that it is expected to be used as a battery substitutive component or assistant component for motor drive. This shows that such a device can not only to back up the memory but also to back up the entire system. Particularly, the electrical double layer capacitor device has been recently developed for power application excellent in large current characteristic because of demand for a power source of electrical vehicle or hybrid vehicle.
By the way, such an electrical double layer capacitor has an application voltage limited below a electrolytic voltage of used electrolyte. The voltage limit is about 1 V when water solution base electrolyte, such as sulfuric acid is used. In addition, it is about 2.5 V when organic solvent base electrolyte, such as propylene carbonate, is used. When a voltage is applied which exceeds this limit voltage , the service life of the capacitor is reduced considerably. Therefore, it is necessary to connect a required number thereof in series when the capacitor is used under an environment over the limit voltage.
However, if a voltage is applied to the capacitor device in which the unit capacitors are connected in series, a disparity takes place in the voltage applied to each unit capacitor owing to the disparity of the capacity and internal resistance of each unit capacitor. Further, this disparity may be increased by means of repeated charging and discharging. As a result, a voltage exceeding the limit voltage is applied to the unit capacitor so that a destruction may be occurred thereof.
As a method of suppressing the disparity of the voltage between the unit capacitors Japanese Unexamined Patent Publication (JP-A) No. SHO 62-4848 (hereinafter referred to as document 1) has disclosed a method of reducing the disparity of the voltage applied to the unit capacitors by connecting a plurality of resistors having a equal resistance in parallel. Further, Japanese Unexamined Patent Publication (JP-A) NO. HEI 6-30247 (hereinafter referred to as document 2) has disclosed a method in which a protective circuit comprises a Zener diode and resistor which are connected in series and is connected to the unit capacitor in parallel.
However, because, according to the method indicated in the above document 1, it is necessary to connect resistances 1/5-1/10 of the insulating resistance of each unit capacitor, respectively as described in the document 1, the leakage current of the entire capacitor is increased five to ten folds as compared to a case in which no resistor is connected. Therefore, a problem exists in which the electric loss is increased tremendously.
Further, because, according to the method described in the above document 2, a current always flows through the protective circuit like the method indicated in the above document 1, a problem is left that power consumption is large. The method of controlling the voltage by such a bypass current is effective when the charging time is very long. However, the by-pass current is increased considerably when charging is carried out in a short time interval of several seconds order. The protective circuit is not practically available by the use of such a large current because not only a power consumption is large but also the parts are very expensive. Further, because the voltages of all the unit capacitor needs to be mentioned according to these control methods by controlling the charging voltage, a problem exists that the protective circuit is complicated and costly.